72 II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



from a solution containing palmitic and stearic acid is ascribable to the 

 same cause. It is claimed by Waentig and Pescheck^*^ that the mutual 

 solubility effect occurs only in solvents such as benzene, carbon tetra- 

 chloride, chloroform, nitrobenzene, and toluene, where an association be- 

 tween the two fatty acids may take place, while no variation is to be noted 

 in benzaldehyde, diethyl ether, ethyl acetate, and ethyl alcohol, in 

 which the acids are known to be monomolecular. Ralston and Hoerr,^^^ 

 on the other hand, believe that the melting point of a given mixture rather 

 than the proportion of bimolecular complex present in a mixture is the 

 determining factor which is related to solubility. 



(4) Fatty Acid Films 



The fatty acids having a chain length of 8 or more carbon atoms are 

 only slightly soluble in water. The solubility which they do possess is 

 largely the result of the carboxyl group, while the hydrocarbon chain has 

 the opposite effect. The longer the hydrocarbon chain is extended, the less 

 the carboxyl group will be able to promote solubility. However, even if the 

 depressing effect on solubility of the hydrocarbon portion of the fatty acid 

 is sufficiently marked as to result in almost complete immiscibility, the 

 carboxyl group will still effect the water-acid interface so that oriented 

 mono- or polymolecular films will be produced. 



a. Monomolecular Films, (a) Nature of Monomolecular Films. The 

 slightly soluble fatty acids have the property of spreading to form a thin 

 and uniform layer when they come in contact with water. This property 

 of forming a film also occurs with immiscible animal or vegetable oils, oxi- 

 dized paraffin oils, fatty acid esters, higher alcohols, other lipids, and re- 

 lated compounds. On the other hand, pure hydrocarbons do not spread 

 when they come in contact with distilled water and so do not possess the 

 ability to form monomolecular films. 



According to Langmuir, ^^'^ the liquids which do not possess the po- 

 tentiality of film formation are substances which contain only nonpolar or 

 hydro-phohic groups. On the other hand, compounds which possess chiefly 

 polar or hydrophilic groups usually enjoy a high solubility and therefore do 

 not form films. It is only those compounds, like fatty acids, which possess 

 both the hydrophilic carboxyl group and the hydrophobic hydrocarbon 

 chain which have the ability to spread over water to form a fine film. 



Most of such films are only one molecule thick. They appear to be 

 oriented in such a manner that the hydrophilic carboxyl groups are dis- 

 solved in the water w^hile the hydrocarbon side chains form a layer on the 

 surface. The molecules are thus aligned parallel to each other and per- 

 pendicular to the surface of the water. 



2« A. W. Ralstou aud C. W. Hoerr, J. Org. Chem., 10, 170-174 (1945). 

 !»« I. Langmuir, J. Am. Chem. Soc, 39, 1848-1906 (1917). 



